The magnetic properties of many ferromagnetic materials undergo changes with stress. For example, the magnetic induction of nickel-iron alloys and iron-cobalt alloys increases and that of nickel decreases with tension stress. Conversely, if these metals are subject to magnetic fields, their dimensions can change. These magnetostrictive effects, including the Joule effect (change in length when a ferromagnetic rod is placed in a longitudinal field) and the Villari effect (change in magnetic condition when a magnetized ferromagnetic rod is subjected to longitudinal stress), can be used for converting electrical power to mechanical power and vice versa. Examples of the use ferromagnetic materials include sensors (U.S. Pat. Nos. 4,414,510 and 5,442,966), transducers (U.S. Pat. No. 3,753,058), and vibrators (U.S. Pat. No. 4,151,432). These types of sensors have low sensitivity ('966 patent) or measure applied magnetic field ('510 patent) rather than stress.
There has been considerable interest in using the magnetomechanical effect in sensors where stress is converted into a change in the magnetization of the magnetostrictive material. Any change in magnetization can be sensed without making contact to the sample. Such a sensor would be ideal for measuring torque in a rotating shaft such as in a drive train or power steering application. There is significant incentive to develop such a torque sensor for power steering applications as the parasitic losses associated with the hydraulic pump in current power steering systems is relatively large. For example, an electronic based system will result in a five percent decrease in fuel consumption.
Terfernol (commercially available as Terfenol D from Edge Technologies, Ames, IA), which is an alloy of terbium, iron and dysprosium, is an excellent magnetostrictive material, however, it has a number of shortcomings. For most applications, Terfernol is not economically viable as a result of the high costs of terbium and dysprosium. Further, in order to obtain optimal results a single crystal is required. As a rare earth intermetallic, Terfenol is extremely brittle and the high content of rare earth metal makes the material extremely susceptible to corrosion.
Other compounds such as nickel and maraging steel have also been considered for use as magnetostrictive material. Nickel has good corrosion resistance and moderate costs, but has only moderate magnetostriction. Maraging steel has a lower magnetostriction than nickel and is lower in cost, but requires carefully controlled heat treatment to produce optimum magnetostriction.
Attempts have been made to use oxide ferrites as magnetostrictive vibrators (U.S. Pat. No. 4,151,432). For example, the '432 patent describes a macroscopically homogenous sintered ferrite structure that is Fe.sub.3 O.sub.4 to Fe.sub.2 O.sub.4.1 either alone or in combination with Fe.sub.2 O.sub.3. These types of compositions have been found to be unsuitable for use in brazing.
It is an object of the present invention to provide magnetostrictive compositions which are effective for use as magnetostrictive sensors and actuators.
It is another object of the invention to provide magnetostrictive compositions with good corrosion resistance and mechanical properties.